Microwave imaging is an area of much current attention for a variety of applications. In the security area the use of microwave and millimeter wave imaging systems offers the potential for providing a non-ionising technique capable of producing high quality images of concealed objects in a fast and inexpensive manner. Most current systems use amplitude and phase measurements using a vector network analyzer to produce images of target objects. In the medical area the ability of microwaves to penetrate to considerable depths beneath the skin in a non-ionising safe manner coupled with the large differences in material properties between healthy and malignant tissues has also stimulated much research interest.
Indirect radar holography is a method to construct 2D or 3D images from some intensity measurements. The advantage of this principle over direct holography is that only a simple diode detector for power measurement is needed to measure the output of the antenna and not a vector analyzer (to measure amplitude and phase). In indirect holography, the intensity pattern of the antenna is measured over an area of pre-defined size. The samples of the intensity pattern are taken over a grid of a pre-defined density. Increasing the density of the grid (intensity measurements) increases the resolution and the accuracy of the image. However, increasing the density of the measurements has some disadvantage of causing a delay and increase in complexity in the acquisition process. In case of a single receiving antenna, the antenna has to be moved to more positions, which requires also more hardware precision. In case of an antenna array, achieving better resolution requires a higher number of antenna elements and more samples (measurements) to be taken.
A microwave indirect holographic measuring method is disclosed in WO 02/23205 A1 and in D. Smith, M. Leach, M. Elsdon and S. J. Foti, “Imaging Dielectric Objects from Scalar Intensity Patterns by means of Indirect Holography”, IEEE Antennas and Propagation Society International Symposium, July 2005. A first electrical signal of microwave frequency is provided. A first part of the first signal is directed to a first antenna. Predetermined changes of phase and amplitude are applied to a second part of the first signal to produce a second electrical signal, which is coherent with the first part of the first signal. Microwave radiation is detected at a plurality of locations by means of a second antenna to generate a third electrical signal at each location. The second and third electrical signals are combined to produce a fourth electrical signal. As a result, a hologram of the radiation pattern observed at the sampling antenna can be produced, which avoids the necessity for a network analyser. This reduces the cost of equipment for carrying out the method, and also enables the apparatus to operate over a wider range of frequencies or even simultaneous operation at multiple frequencies, which in turn broadens the range of applications of the method.
However, there is still a need for a technique that accelerates the acquisition process in indirect holography and provides simultaneously high resolution intensity measurements.